The invention was devised to demodulate DQPSK signals in mobile hand-held radios where non-linear receivers are used because of the need to limit power consumption.
From first principles, the optimum solution for demodulating a DQPSK signal is to employ a square-root Nyquist filter at the receiver followed by a coherent detector, and under Rayleigh fading environment non-coherent detection (or differential detection) has often been proved to give the best performance. However, this optimum solution requires the use of linear amplifiers in the receiver, and linear amplifiers consume more power than non-linear log-amplifiers with today's technology. This power consumption problem is particularly true for the Japanese Digital Cellular (JDC) system where, due to the use of a post-detection diversity technique, a dual-receiver structure is needed. A more practical solution for hand-held units here is to adopt one of the conventional techniques for narrowband FM signals where the receiver uses a limiter IF amplifier followed by an analogue frequency discriminator detector.
This is shown in FIG. 1 of the accompanying drawings where the intermediate frequency signal 10 (which consists of a sinusoidal carrier modulated by DQPSK so as to carry transmitted symbol values) is fed to a limiter amplifier 12 and thence to an analogue frequency discriminator 13 having a delay line 14 and a low pass filter 15. The discriminator 13 produces a frequency deviation signal which needs to be integrated over each symbol period to derive an output signal representative of the carrier phase change and, therefore, the transmitted signal. The output of the analogue frequency discriminator 13 is converted to digital format in an analogue to digital converter 16 and then fed to digital hardware 17, in which the signal is integrated and dumped into one of four decision bands depending on the modulation phase which is representative of the transmitted symbol value.
The logarithm of the amplitude of the signal 10 is derived from the limiter amplifier 12 and is fed, via a further analogue to digital converter 18, to the system hardware 17. This is done to deal with antenna diversity, the receiver having two antennas and the system automatically choosing the stronger of the two received signals.
Although discriminator detection provides an almost optimum performance for narrow-band FM, it does not work so well with QPSK modulation, since the latter is not a continuous phase modulation, containing large frequency deviations and requiring a wide band frequency discriminator circuit. Computer simulation results have shown that performance loss of up to 1 dB may be caused by the imperfection of the practical analogue discriminator circuit, both in the Static and Rayleigh fading with diversity. Furthermore, component variation and imbalance in the analogue circuit introduce large DC offset and signal amplitude variation which are difficult to compensate, leading to further performance loss and increase in circuit complexity.